Means for retaining gas-filled vial in pre-tr unit

ABSTRACT

A TR-limiter apparatus includes a pre-TR unit for response to RF energy of a predetermined minimum level. The unit includes a gasfilled vial mounted between a resonant cavity and a slot for coupling RF energy below the predetermined level to the cavity and for reflecting RF energy above the predetermined level. A resiliently biased member urges the gas-filled vial firmly against a retaining surface of a housing to maintain the vial at the inner end of a coupling slot through which the RF energy is supplied. In this manner, the recovery time of the pre-TR unit is minimized and the resonant frequency of the cavity is stabilized despite thermal cycling of the vial and the housing in the operation of the TR-limiter apparatus.

United States Patent [191 Prescott [451 Aug. 14, 1973 [75] Inventor: Thomas Gates Prescott, Wenham,

Mass.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: June 29, 1972 [21] Appl. No.: 267,282

Primary Examiner-Paul L. Gensler Attorney-C. E. Martine, Jr. et al.

[5 7 ABSTRACT A TR-limiter apparatus includes a pre-TR unit for response to RF energy of a predetermined minimum level. The unit includes a gas-filled vial mounted between a resonant cavity and a slot for coupling RF energy below the predetermined level to the cavity and for reflecting RF energy above the predetermined level. A resiliently biased member urges thegas-filled vial firmly against a retaining surface of a housing to maintain the vial at the inner end of a coupling slot through which the RF energy is supplied. In this manner, the recovery time of the pre-TR unit is minimized and the resonant frequency of the cavity is stabilized despite thermal cycling of the vial and the housing in the operation of the TR-limiter apparatus.

6 Claims, 4 Drawing Figures 2 Sheets-Sheet 1 050 FORCE ANGLE .070 FORCE ANGLE 52 I v .090 FORCE ANGLE A .IIO FORCE ANGLE DIODE TR PRE-TR' RECEIVER mm TUBE UNIT TRANSMITTER A f 22 I8 N2 Patented Aug. 14, 1973 3,753,158

2 Sheets-Sheet 2 FIG.3

MEANS FOR RETAINING GAS-FILLED VIAL. IN PRE-TR UNIT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to TR-limiter apparatus, and more particularly to a pre-TR unit connected in series with a TR tube, a diode limiter, and a return signal receiver for limiting the power applied to the receiver.

ln the field of radar, for example,,T.Rlimiter apparatus is used to prevent output pulses from a transmitter from being received directly by the return signal receiver. Depending upon. the power level of the output pulses, the TR-limiter apparatus may include a diodelimiter for response to low average power pulses, a TR tube for response to mediumaverage power pulses, and a pre-TR unit. for response to high. average power pulses.

The recovery time of the TR' tube or the pre-TR unit is the time between the end of an output pulse and: the instant at which the TR tube or pre-TR unit is in condition to passa return. signal to the receiver. Obviously, the recovery time should: be as short as possible sothat the entire return signal from each: output pulse. is. applied to the receiver. In. a. TR' tube or. other gas-filled device inwhichthe gasionizesin ICSPOHSClZOflIIEOUlPUi pulse, the recovery. time isthe timerequiredforthe gas to de'ionize once the output. pulse stops.

2. Description of the Prior Art In the past, pre-TR unitslocated betweenthe transmitter and the receiver have beenmade from vacuum tight enclosures containing gas that ionizes in response to each output pulse andtthusprotects the receiver'. To simplify the pre-TRi units and permit use of lessiexpensive castings, the gas has, inthepast, beencontained in a thin-walled vial fabricated from quartz,,for example, or another material having a low dielectric constant and the ability to withstandoperatingtemperaturesup:

to 100 C, for example. The'outer cylindrical surface of the vial has been ground to a uniformdiameter and the vial received in an accurately, machinedscylindricalaperture providedin a housing. A wall having athickness of about 0.025 inches extends betweenaretaining surface of the aperture and an outside surface of the housing. A coupling slot extends from the outside surface has been inserted in-each endof the aperture. to prevent axial movement ofJthevial in the aperture.

The above-described structureof thepre-TR unit has been the source ofa numberof problems. In particular, the quartz vial adjacentto the coupling slot changes the. resonant frequency of the cavity. It has been foundthat this resonant frequency varies according to the intimacy of the contact of the vial with the inner end of the coupling slot. Because the diameter of! the aperture cannot practically be machinedwith exact accuracy, a 1 to 3 mil gap may exist between the inner end of the;

coupling slot and the outer surface of the vial. To eliminate this gap, the upset lip must urge the vial against the coupling slot of the aperture. However, it is difficult to form the lips in this manner and the upsetting tooloften breaks thevial. Further, most of the force of the lips on thevial is applied in a direction perpendicular to rather than toward the coupling slot which tends to crack the vial without retaining it in the aperture. Also, because the aperture diameter increases more than the vial diameter during operation, the lips permit the via] to become loose and move away from the inner end of the SUMMARY OF THE INVENTION The present invention. avoids the. disadvantages of the prior technique for holding the vial. in the aperture. of thepre-"ITR unit while increasing the range of energy of output signals towhich the pre I R section is responsive. In particular',,without increasing the requirements.

of accuracy for the fabricationof the vial aperture or the outer: diameter of the vial, the present invention includes facilities for'firmly holding: the vial against the portion of the vial'aperture thatis adjacent to the couplingslot. In greater detail, such facilities are effective tohold the viall firmly against such portion of the vial aperture during. operation of the pre-TR unit even though the vial and the aperture expand different amounts-in= response to temperature variations accompanyingoperation of the pre-TRunit As a result, the recoveryxtime'of thepre-TR unit of the present'invention. is stabilized and variations in the resonant frequency slot are minimized. Further, breakage of vials duringthe process of securing the vial in the aperture hasbeenreduced to insignificant proportions.

Anobjectof this-invention is to provide a new and improvedTR-limiter apparatus.

Anotherobject of. the present invention resides in minimizing the recovery time of a gas-filled vial mounted in'a pre-TR unit of aTR-Limiter apparatus.

Afurther object'of the present invention is toprovide facilities in a pre-TR unit for firmly mounting a gas filledvialadjacent to a couplingtslot throughwhich RF energy is admitted.

Astill further object of the present invention resides in an aperture provided in a housing of apre-TR unit forreceiving. a gas-filled vial that is engagedby a resiliently biasedretainer having a vial engaging end section conformingtothe shapeofthe vial for holdingthe vial firmly in position adjacent an interior end of aslot through which RFenergy is coupled.

With theseand other objects in view, the present invention contemplates the use of a. gas-filled vialin a pre-TR unit for protecting a radar return signal receiver froml'lighenergy output (or radar) pulses. The gas-filled vial is mounted in a first aperture extending parallelto and closely spaced from an endsurface of a housing. of the pre-TR unit. Aslotfor coupling the output pulses into the aperture extends through a portion of the wall between the first aperture and the end surface. A second aperture provided in the housing extends parallel to the end surface and intersects the first aperture. A retaining member slidably received in the second aperture is urged against the vial by a resilient member. The end of the retaining member that engages the vial is shaped to conform to the shape of the outer surface of the vial. The retaining member is dimensioned and the second aperture is located relative to the first aperture such that the retaining member exerts a resultant force on the vial that includes a force component in the direction of the coupling slot and perpendicular to the longitudinal axis of the first aperture so that the vial is held firmly in the aperture notwithstanding size variations of the vial and dimensional variations of the aperture which occur during high temperature operation of the pre-TR unit. The length of the retaining member and the shape of the vial engaging end thereof are designed to minimize the electrical effect of the retaining member on the operation of the pre-TR unit while enabling the retaining member to maintain enough component force on the vial to firmly hold the vial in the aperture.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and an understanding of the present invention may be obtained upon reference to the following dcscription of a preferred embodiment thereof when taken in conjunction with the appended drawings in which:

FIG. 1 is an outline drawing of a TR limiter apparatus including a pre-TR unit connected to a transmitter for preventing radar output pulses from being conducted to a receiver;

FIG. 2 is an end view of the pre-TR unit located between the transmitter and a TR tube illustrating a quartz, gas-filled vial secured in place by a resiliently biased retaining member;

FIG. 3 is a cross-sectional view of the pre TR unit taken along line 33 in FIG. 2 showing the retaining member urging the gas-filled vial to the left against a retaining wall of an aperture so that the vial is urged into intimate contact with an inner end of a coupling slot; and

FIG. 4 is a graph illustrating the relationship between the angle at which the retaining member applies a resultant force to the vial and the length of the retaining member.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a TR-limiter apparatus l constructed according to the principles of the present invention is shown in FIG. 1 including a tranS- mitter 12 for generating a series of output pulses 14, such as those used for radar purposes. In such applications, a target (not shown) reflects the output pulses l4 and generates a return signal 16 which is conducted by a pre-TR unit 18, a TR tube 20, and a diode-limiter 22 to a receiver 24. The pre-TR unit 18, the TR tube 20, and the diode-limiter 22 are also effective to isolate the receiver 24 from the output pulses 14 so that the amount of power fed to the receiver 24 is limited to the power level for which the receiver is designed.

Referring now to FIG. 2, the pre-TR unit 18 is shown in detail including a housing 26 which may be fabricated from a casting, such as of aluminum. As shown in FIGS. 2 and 3, the housing 26 has a generally rectangular cross-section and is provided with spaced end surfaces 27 and 28. The surface 27 is mounted against a waveguide (not shown) of the transmitter 12, whereas the surface 28 is mounted against the TR tube 20. A first, generally rectangular aperture 29 is provided in the housing 26 and extends (in the direction of an axial line 30) from the surface 28 for defining a resonant cavity 31. The cavity 31 is provided with side walls 32, an end wall 33 and respective top and bottom walls 34 and 36.

A second or vial aperture 37 is provided in the housing 26 and extends generally perpendicular to the line 30 along an axis 38. The second aperture 37 is shown having a cylindrical cross-section, although other crosssections would be suitable. The second aperture 37 intersects the end wall 33 of the cavity 31 such that a portion 41 of the second aperture 37 that is co-extensive with the cavity 31 is provided with a generally semicircular cross-section for defining a retaining surface 42.

The housing 26 is further provided with a third aperture 43 extending generally parallel to the surfaces 27 and 28 and intersecting the bottom wall 36 of the cavity 31 adjacent to the retaining surface 42. In addition, a slot 44 is provided in the housing 26 extending from the surface 27 through a wall 46 that extends between the end surface 27 and the retaining surface 42 for permitting the pulses l4 and the signal 16 to enter the second aperture 37.

As shown in FIGS. 2 and 3, a vial 47 having an outer surface 48 shaped to conform to the shape of the second aperture 37 is received in the housing 26 with the surface 48 touching the retaining surface 42. The vial 47 may suitably be fabricated from quartz although materials other than quartz may be selected if they have a suitably low dielectric constant and a similar ability to withstand the relatively high temperatures encountered during the operation of the TR limiter apparatus 10. The vial 47 is sealed for enclosing a gas, such as chlorine, which becomes ionized to form a plasma discharge in response to a pre-determined amount of energy from an output pulse 14. When the gas is ionized, an output pulse 14 will be reflected to isolate the receiver 24 from the output pulse 14.

Although the diameter of the second aperture 37 is carefully machined so as to maintain accurate tolerances, it is generally not practical to machine the second aperture 37 such that the diameter thereof is substantially identical to that of the outer surface 48 of the vial 47 which is ground to a relatively constant diameter. As a result, if the vial 47 is not held in place, it is usually free to move to a limited extent, such as 1-3 mils, relative to the retaining surface 42.

As noted above, prior techniques for securing the vial 47 against the retaining surface 42 of the second aperture 37 have resulted in variations of the resonant frequency of the cavity 31 in an uncontrolled amount. In addition, it has been found that certain movement of the vial 47 permitted by such prior methods results in a measurable increase in the time required for the gas in the vial to de-ionize after the output pulse 14 stops. In particular, the recovery time of prior pre-TR units is significantly increased when the vial moves away from an inner end 51 of the coupling slot 44.

The pre-TR unit 18 of the present invention avoids the problems associated with such prior techniques by providing in the third aperture 43 a slidable retaining member 52 having a first end 53 provided with a reduced diameter section 54, an intermediate section 56 having a diameter slightly less than that of the third aperture 43 and a third, inner end section 57 having a surface 58 thereon having a shape that conforms to the shape of the outer surface 48 of the vial 47. As shown in FIG. 3, when the vial 47 has a circular cross-section, the surface 58 of the member 52 that engages the vial has an arcuate contour conforming to that of the outer surface of the vial. The first end 53 of the member 52 is shown surrounded by a resilient member 61, such as a coil spring, which extends further outwardly in the third aperture 43 to a set screw 63 which is received in a threaded section 64 of the third aperture 43. The position of the set screw 63 in the third aperture 43 is adjusted to compress the coil spring 61 so as to urge the member 52 into engagement with the outer surface of the vial 47. The resultant force (shown by arrow 65) applied by the retaining member 52 against the vial 47,

has a retaining or horizontal component (shown by arrow 66) extending to the left as viewed in FIG. 3. This retaining force component 66 is significant enough (such as 2 ounces) to maintain the vial firmly in contact with the retaining surface 42 so that a section 67 of the outer surface of the vial 47 is maintained adjacent to the inner end 51 of the coupling slot 44. In this manner, the vial 47 is held in a pre-determined location such that once the resonant frequency of the cavity 31 has been adjusted (by. means of a tuning rod 68 shown in FIG. 3) the resonant frequency of the cavity remains constant.

It may be appreciated that-the coefficients of thermal expansion of the quartz vial 47 and the aluminum housing 26 are different. Thus, the diameter of the aperture 37 and that of the vial 47 do not change at the same rate upon heating during operation of the pre-TR unit 18. The resiliency of the coil spring 61 compensates for differences in the size of the second aperture 37 and the vial 47 caused by heating or by machining tolerances by maintaining the section 67 of the vial 47 in contact with the retaining surface 42. In this manner, the resonant frequency of the cavity 31 is stabilized and the gas discharge within the vial (that results from a pre-determined amount of energy coupled through the coupling slot 44). is of constant density so that the recovery time for the pre-TR unit 18 does not vary.

Referring now to FIG. 4, there is shown a graph indicating the relationship between the length of the retaining member 52 as a function of the force angle or angle at which the resultant force 65 is applied by the retaining member 52 to the outer surface 48 of the vial47. Superimposed on the graph is an outline of the retaining member 52 viewed as in FIG. 3 and illustrating side surfaces 71 and 72 of the retaining member 52 along with the arcuate surface 58 which conforms to the shape of the outer surface 48 of the vial47. Dashed lines 73, 74, 76, and 77 represent various end surfaces of the member 52 and thus illustrate varying lengths which the member 52 may have. For purposes ofconvenience, the lines 73, 74, 76, and77 are shown measured downwardly from a horizontal line 78 passing through the centerline of the vial 47. Thus, the line 73 representing the longest length of the retaining member 52 is indicated as being 0.05 inches from center line 78 and the end line 74 is indicated as being 0.07 inches from the center line 78.

Referring again to FIG. 4, the graph also shows a series of lines 81, 82, 83, and 84 which pass through the intersection of center lines 78 and 86. The intersection of the lines 78 and 86 defines the longitudinal axis 38 of the vial 47. The angle A between the horizontal center line 78 and the lines 81, 82, 83, and 84 indicates the angle of the resultant force 65 applied by the various retaining members 52 on the vial for the lengths indicated by respective lines 73, 74, 76, and 77.

In general, the force angle lines 8l-84 indicate that as the length of the retaining member 52 is decreased, the horizontal (or leftward) retaining component of force 66 decreases, thus there is less force to retain the via] 47 against the retaining surface 42. In addition, it may be understood that for a given diameter retaining member, as the length of the retaining member 52 decreases, the area of contact between the retaining member 52 and the vial 47 decreases such that for a given resultant force 65 applied to the vial, the pressure on the vial 47 increases. Also, it may be appreciated that the more the retaining member 52 extends into the cavity 31, the greater the reactive effect of the member 52 on the resonant frequency of the cavity 31. Because the ideal length of the retaining member from a reactive standpoint would result in a negligibly small retaining force and a substantial increase in the pressure applied to the vial 47, the length of the retaining member 52 is selected so as to provide an adequate retaining force 66, and a relatively large contact area while being short enough to have a reactive effect on the resonant frequency that can be compensated for by adjustment of the tuning screw 68. In a particular embodiment of the present invention, the length of the retaining member 52 as measured from the line 78 was 0.07 inches.

Tests conducted with the pre-TR unit 18 of the present invention confirm the stabilization of the resonant frequency of the cavity 31 and the minimization of the recovery time of the pre-TR unit. However, an additional unexpected result of the use of the retaining member 52 of the present invention is that the level of RF power required to ionize the gas in the vial 47 is significantly reduced, apparently because the retaining member (when fabricated from aluminum, for example,) acts as a high potential point. The high potential is thought to cause the gas to ionize at a lower level of incident of RF energy, thus providing an increased range of protection to the TR tube 20.

It may be understood that the principles of the present invention may be used by those in the art in em bodiments other than that disclosed above. For example, a pair of retaining members 52 mounted on opposite sides of the vial 47 may be used to apply increased components of retaining force 66 to the via] 47 for added security in holding the vial against the retaining surface 42.

What is claimed is:

1. Apparatus for limiting the electrical power of a transmitted signal supplied to a receiver, comprising:

housing means for defining a cavity, said housing being provided with a first surface and a first aperture intersecting said cavity, said aperture including a retaining surface extending adjacent to said first surface so as to define a wall bounded by said first surface and said retaining surface,

said housing means being provided with a slot having an inner end adjacent to said aperture for coupling said transmitted signal to said aperture;

gas discharge means received in said aperture in said housing means;

means slidably mounted in said housing means and movable into said cavity for engagement with said gas discharge means; and means for urging said slidable means into engagement with said gas discharge means to press said gas discharge means firmly against said retaining surface and adjacent to said inner end of said coupling slot.

2. The apparatus of claim 1, in which:

said gas discharge means is a gas-filled vial,

said slidable means is received in a second aperture provided in the housing means on the side of the first aperture opposite to said retaining surface so that said slidable means is effective to press said vial against said retaining surface and said slidable means is provided with a surface having a shape conforming to the shape of the outer surface of said vial for reducing the pressure applied to the outer surface of said vial.

3. The apparatus of claim 1, in which:

said cavity and first and second apertures are mutually perpendicular; and,

said gas discharge means is a gas-filled vial; and

said slidable means engages one side of said vial for exerting a force on said vial, said force having a component directed toward said coupling slot for maintaining the side of said vial opposite to said one side against said retaining surface of said aperture.

4. A pre-TR device adapted to limit the power of a signal applied to a receiver, comprising:

a housing having a first surface and a cavity, said housing including a slot extending through said surface toward said cavity for coupling a signal having a maximum selected power level to said cavity, said housing being provided with a first aperture extending along said surface and intersecting and extending between both said coupling slot and said cavity;

a gas-filled vial received in said aperture for limiting the power level of the signal received by said cavity from said coupling slot to said selected power level;

a member having a first end provided with a shape conforming to the shape of the outer surface of said vial for contacting said vial; and

means for resiliently urging said member against said vial to maintain said vial against the wall of said first aperture adjacent to said coupling slot.

5. The apparatus of claim 4, in which:

said housing is further provided with a second aperture intersecting said cavity,

said member is slidably received in said second aperture for movement into engagement with the outer surface of said vial, and

said first end of said member is designed to extend a minimum distance into said cavity and to exert a force on said vial at least a component of which is in the direction of said coupling slot for maintaining said vial against said wall of said aperture.

6. The apparatus of claim 4, in which:

said cavity and said first aperture intersect so that at said intersection said first aperture has a generally semi-circular cross-section,

said vial is mounted in said aperture and extends into said cavity at said intersection; and

said urging means presses said retaining member into engagement with the portion of the vial that extends into the cavity so that the vial is maintained against said wall of said aperture adjacent said coupling slot. 

1. Apparatus for limiting the electrical power of a transmitted signal supplied to a receiver, comprising: housing means for defining a cavity, said housing being provided with a first surface and a first aperture intersecting said cavity, said aperture including a retaining surface extending adjacent to said first surface so as to define a wall bounded by said first surface and said retaining surface, said housing means being provided with a slot having an inner end adjacent to said aperture for coupling said transmitted signal to said aperture; gas discharge means received in said aperture in said housing means; means slidably mounted in said housing means and movable into said cavity for engagement with said gas discharge means; and means for urging said slidable means into engagement with said gas discharge means to press said gas discharge means firmly against said retaining surface and adjacent to said inner end of said coupling slot.
 2. The apparatus of claim 1, in which: said gas discharge means is a gas-filled vial, said slidable means is received in a second aperture provided in the housing means on the side of the first aperture opposite to said retaining surface so that said slidable means is effective to press said vial against said retaining surface and said slidable means is provided with a surface having a shape conforming to the shape of the outer surface of said vial for reducing the pressure applied to the outer surface of said vial.
 3. The apparatus of claim 1, in which: said cavity and first and second apertures are mutually perpendicular; and, said gas discharge means is a gas-filled vial; and said slidable means engages one side of said vial for exerting a force on said vial, said force having a component directed toward said coupling slot for maintaining the side of said vial opposite to said one side against said retaining surface of said aperture.
 4. A pre-TR device adapted to limit the power of a signal applied to a receiver, comprising: a housing having a first surface and a cavity, said housing including a slot extending through said surface toward said cavity for coupling a signal having a maximum selected power level to said cavity, said housing being provided with a first aperture extending along said surface and intersecting and extending between both said coupling slot and said cavity; a gas-filled vial received in said aperture for limiting the power level of the signal received by said cavity from said coupling slot to said sElected power level; a member having a first end provided with a shape conforming to the shape of the outer surface of said vial for contacting said vial; and means for resiliently urging said member against said vial to maintain said vial against the wall of said first aperture adjacent to said coupling slot.
 5. The apparatus of claim 4, in which: said housing is further provided with a second aperture intersecting said cavity, said member is slidably received in said second aperture for movement into engagement with the outer surface of said vial, and said first end of said member is designed to extend a minimum distance into said cavity and to exert a force on said vial at least a component of which is in the direction of said coupling slot for maintaining said vial against said wall of said aperture.
 6. The apparatus of claim 4, in which: said cavity and said first aperture intersect so that at said intersection said first aperture has a generally semi-circular cross-section, said vial is mounted in said aperture and extends into said cavity at said intersection; and said urging means presses said retaining member into engagement with the portion of the vial that extends into the cavity so that the vial is maintained against said wall of said aperture adjacent said coupling slot. 